In the "old" days, cats would be put outside at night to carry out it's rodent extermination duties but also to control the amount of mess it creates in the house. In the modern day, we have effective rodent control and also very effective litter and litter trays rendering letting the cat out obsolete.
A cat's freedom is now more likely to be dependent on the human carer and their life schedules. Unless you have specific circumstances which necessitates an indoor lifestyle for your cat, it is quite common to give your pet various degrees of access to the outside world.
Some cats will automatically confine their territories to the back garden whilst others may develop an awareness of the traffic in the area and avoid rush hours. That said, it is still very difficult to know the abilities of your cat and thus gauge the level of danger it will present itself with if it goes outside. For instance, some quiet roads can still be dangerous because of the odd unexpected cars. Territorial fighting can expose your cat to infection and together with the earlier mentioned reasons, it is therefore wise to exercise some form of control over your cat's freedom to roam.
This is where cat flaps help a great deal.
Cat flaps are effectively cat sized windows which are fitted to the base of a door. The most practical design is one which works with gravity such that the door automatically shuts after entry or exit, and, it must have a plastic clear window so that the cat can look outside before venturing out.
The main disadvantage of cat flaps is that feral cats and neighbours cats will also learn to use them without constant human observation. This again could lead to disease or infection being brought into the home as well as fights.
One way to overcome this is by using smart cat flaps that only allow entry to cats wearing the appropriate collar and gadgetry. Alternatively, you can allow your cat limited freedom by locking the cat flap up at night, or when you are unable to keep an eye on unwanted visitors. In any case, your cat should soon acclimate to your routine.
When introducing your cat to the cat flap, spend time encouraging it and showing it how the door works. Put your cat on one side, and call encouragingly from the other. After a few tries, it will know what to do!